Electron lenses



Feb. 23, 1960 D. GABOR ELECTRO LENsEs ymed June 1a, 1956 Sheets-Sheet 1 l FIGA.

INVENTOR DENNIS GABoR ATTORNEYS Feb. 23, 1960 D. GABOR 2,926,274

ELEc'rRoN LENsEs Filed June 18, 1956 4 Sheets-Sheet 2 ATTORNEYS 4 Sheets-Sheet 3 Filed June 18, 1956 I FIG.8.

INVENTOR oENms GAaoR ATTORNEYS Feb. 23, 1960 D. GABOR 2,926,274

ELECTRON LENsEs I 4 Sheets-Sheet 4 Filed June 18, 1956 INVENTOR DENNns GABOR By @cz/"Lm, @a/pm AT TORNEYS United States Patent l ELEcrRoN LENsEs Dennis Gabor, London, England, assignor to National Research Development Corporation, L'ondon, England, a corporation of Great Britain Application June 18, 1956, Serial No. 592,142 Claims priority, application Great Britain June 20, 1955 14 Claims. (Cl. 313-77) This is a continuation-impart of application Serial No. 309,677, filed on 15th September, 1952, now abandoned, and -application Serial No. 549,712, filed on November 29, 1955, now Patent No. 2,795,729, issued June 11, 1957. These applications describe and illustrate various forms of cathode ray tube having the feature that `they .can be made flat in shape, that is to say, without any appreciable depth perpendicular to the screen, the conical and tubular neck porltlions of conventional cathode ray tubes being dispensed wit `In one form of such a cathode ray tube, the structure is housed in an envelope the shape of which may be-regarded as resembling that of a hand mirror. In a preferred arrangement, however, a more compact structure is achieved by doubling the electron beam backv upon itself by meansv of an electron lens adapted to receive a beam deflectable in one plan-e and deliver it baf'ck in another plane parallel -to the first but displaced therefrom.

One object of .the invention is to provide an electron lens adapted to effect such a reversal of an lectron beam.

Another object of the invention is to provide a reversing lens for use in a flat cathode ray tube as above described, which will enable the deflection angle, through which the beam must be deflected for producing a television line scan in a cathode ray tube of the kind above described, t be materially reduced.

A further object of the invention is to provide a reversinglens for the purpose of reversing yupon itself a cathode ray beam deflectable in a given plane, which will enable certain aberrations, due to such deflection of the electron beam, to be reduced or eliminated.

Yet another objection of the invention is to .provide a cathode ray tube which will be compact and not greatly exceed in any of its `dimensions the dimension of the picture screen.

Further objects will appear as the description proceeds.

I'n the cathode ,ray tubes described in my prior application Serial No. 549,712 the picture producing electron beam is first defiected in one direction only, parallel to the screen surface, and then proceeds in a direction which is also substantially parallel Vto the screen, at a relatively small distance from its surface. When the beam reaches a certain zone, it is strongly deflected towards the screen by a locally applied electric eld, so that it forms a curved pointer, meeting the screen at the desired point. This localized electric field extends over a substantially linear zone which always intercepts the beam, irrespective of its first mentioned deflection, and is shaped in'rsuch a way that the beam is focused by said field at the same time as it is deflected thereby. In the application of the invention to television, the linear zone of the electric field is in the direction of the line scan, while the frame scanning is achieved by moving said field parallel to itself, in the direction of the frame scan. It follows that, as the beam moves substantially parallel to the screen and close to it, the tube can be made very fiat. No throw at right angles to the screen is'required, as in all conventional cathode ray and television tubes.

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The functional principle of the tube described in that prior application may be translated into structural terms in the following manner. Within a glass envelope there is provided a substantantially plane fluorescent picture screen adapted to be maintained at a maximum positive potential of the order 5-15 kv. An array of linear conductors is arranged in a plane parallelto the screen, the conductors being substantialy parallel to one another and to the direction of the line scan, insulated from one another and preferably associated withv a common capacitive backing plate. This array of conductors may be called the scanning array. An electron gun is positioned in the envelope and arranged so as to project an electron beam into the space between the scanning array and the screen. If the array is charged up to a maximum positive potential, equal to the potential of the fluorescent screen, there is no electric field between said array and the screen, and the cathode ray beam will pass through between them without being deflected. If, however, a zone of the array is discharged, so that in the zone successive conductors assume a graduated range of potentials extending from the maximum positive potential down to 'a potential in the neighborhood of that of the cathode of the electron gun, the electrons will Ibe repelled by said zone and thrown towards the fluorescent screen. lf the conductors of the array are discharged, one by one, progressively across the array in the direction towards the electron gun, the zone will travel across the array so that the beam will be thrown on the screen after a progressively diminishing length of travel, and this effect may be used to provide the frame scan for a television picture presentation ernploying the tube.

The discharging of the conductors to effect the frame scan may be achieved in any suitable mannerfas by a special organ, called the scanning valve, contained :in the vacuum space of the tube itself. The scanning valve, which forms no part of the present invention, is described and illustrated in my application Serial No. 549,712 filed November 29, 1955.

In one form of the device the electron beam is directed into the space between the screen and the array .of conductors which perform the forward deflection from an electron gun positioned below the screen, and the whole is enclosed in an envelope which is of a shape referred to above as resembling -that of a hand mirror. In the more compact arrangement which has also been referred to, however, the gun is placed behind the array of conductors and is projected downwardly, and is then turned back upon itself around the bottom edge of the array lof conductors so as to enter in an upwards direction the space between the conductors and the screen. For this purpose an electron-lens according to the present invention is used. The lens has a trough-shaped repeller electrode normally maintained at low potential (e.g. the cathode potential of the electron gun), a central or spine electrode maintained at high positive potential, and two side or flanking electrodes also maintained lat high positive potential. The electron beam enters between the spine electrode and one of the flanking electrodes, is bent round the edge of the spine electrode back upon itself, emerging between the spine and the other anking electrode. The lens may be regarded as a cylindrical lens having a U-shaped optical axis.

The invention will be better understood from the following description given with reference to the accompanying drawings in which:

Figures l, 2, 3 and 4 are a front view, a side view, a rear view and a plan view, respectively, all partly sectioned and somewhat diagrammatic in character, of one form of cathode ray tube embodying the invention.

Figure 5 is an enlarged vertical section through the electron lens used in the tube illustrated in Figures 1-4.

Figure 6 is a cross section of a modified reversing lens according to the invention.

Figures 7 and 8 are an elevation, partially broken away and an elevational cross section, respectively, of a curved reversing lens according to the invention, in association with a pair of deflector electrodes.

Figure' 9 is a perspective view of a deflection system for use in conjunction with a reversing lens according to this invention.

Figure is a perspective view of a modified spine electrode for a lens according to the invention, and

Figure 11 is a perspective view of a modified repeller electrode for a lens according to the invention.

In Figures 1-4 the vacuum envelope 1 consists of glass on the side from which the screen 2 is viewed, while the other parts may be made of glass or metal. The fluorescent screen 2 is in the form of a phosphor coating on a sheet of suitable material such as glass, or glass cloth suitably tensioned, and may be backed with the now widely used metallic layer. Facing the screen, and at a relatively small distance from it, is arranged the scanning array 3, details of which are given in my application Serial No. 549,712 filed November 29, 1955. rIlhe array is backed, at least in part, by the metal plate 4, which may be used as support for the rear array but is insulated therefrom and constitutes the common capacitive backing of the array. As described in the aforesaid application, the conductors of the array 3 may be folded over in a loop 5 to form front and rear arrays, and also folded around the backing plate 4 and into a loop 6 which forms the outer electrode of the scanning valve, whose grid 7 and collecting electrode 8 are also shown. Tlhe scanning valve forms no part of the present invention and need not therefore be described in detail here.

The electron beam E is formed by the electron gun 9, which may be of conventional design, and is not therefore shown in detail.

The gun 9 is placed behind the scanning array 3 and positioned in the vertical plane of symmetry perpendicular to the screen and pointing vertically downwards. At the bottom of the tube the beam from gun 9 iS reversed upon itself so as to pass upwards between the array 3 and screen 2, 'the reversal of the direction of the beam being effected by means of an electron lens formed of elements 13, 14 and 15 to be described in more detail later.

The line scan, that is to say the deflection horizontally across the iiuorescent screen as shown in Figure l, is effected by electrostatic deection plates 16, 16.

Two pairs of small deector electrodes 11 and 12 are provided which operate in opposition to one another so as to displace the beam parallel to itself. For a tube operating only in black and white these electrodes are given fixed potentials adjusted to direct the electron beam exactly into the narrow gap between the screen 2 and the array 3, thus correcting for small inaccuracies in the mounting of the electron gun relative to said gap. In color tubes they have the additional function of color control.

It is known that electrostatic deflection is rather unsuitable for wide scanning angles, but in view of the long throw of the beam achieved by bending it back upon itself, the scanning angle can be reduced to a low value; 19 in the example illustrated. It can be still further reduced by adopting a modified form of reversing lens to be described later.

As shown in Figures 1 and 3, the effect of the electron lens 13, 14, 15, viewed in the plane of these drawings, is the same as specular reflection on a line M-M, which is Well outside the physical boundaries of the lens. Thus the deectors 16 must be so adjusted as to scan the mirror image of the fluorescent screen relative to said line M-M, as illustrated in Figure 3. It will be clear that trapezium correction must be applied to the scan, and a corresponding correction applied to the focus, at least in the plane of Figures 1 and 3. Such techniques have been employed in the operation of previously known cathode ray tubes in which the screen is inclined relative to the tube axis. On the other hand, no keystone correction need be applied in the present case because the line in which the electron beam meets the screen is substantially predetermined by the deflecting zone, set up by the deflection array, which is straight.

This type of tube is eminently suitable for manufacture, because the fluorescent screen, the scanning array, the backing plate, the gun and dellector system and the electron lens or mirror may be fixed relative to one another by suitable beams, struts and spacers, well known in the art of electron tube manufacture, so as to form one cornposite structure. The envelope 1 is preferably made of two dish-shaped halves, divided in the symmetry plane S-S as shown in Figures 2 and 4, and the composite structure above referred to is sealed between them, preferably supported by studs which project between the two half-dishes, in pre-moulded grooves, not shown in the drawings. The electrode leads are also preferably sealed in at the joint between the two halves. Thus great accuracy of manufacture can be achieved, because the internal structure can be completely assembled in jigs, and the relative positions of the parts are not affected by the subsequent glass sealing operations. The most critical adjustments are Ithose which affect the parallelism of the beam and the liuorescent screen. These, however, can be made after the tube is completed. If the plane in which the beam is scanned for the line scan happens to be tilted around a horizontal axis, so that it runs at an angle instead of parallel to the screen, the bias of the trimming deflectors 11 or 12 may be slightly altered to correct it. lf this plane is twisted round a vertical axis, e.g. by imperfect positioning of the defiectors 16, the twist can be compensated by a weak vertical magnetic field, produced by a few turns of a current-carrying wire wound round the tube, say at the level of the electrodes 13. The electron lens or mirror 13, 14, 15 is shown in one form in detail in Figure 5. In the form illustrated in this figure the lens comprises two cylindrical electrodes 13, 13 hereinafter called flanking electrodes placed one on each side of a third electrode 14, which will be called the spine, also cylindrical, located in the symmetry plane S-S. All three of these electrodes are connected to a positive potential, preferably the highest positive potential in the tube, Vm. Associated with electrodes 13, 13 and 14 is a hollow cylindrical electrode or trough 15 termed the repeller electode, which is positioned symmetrically beneath the other electrodes and is kept at or near the potential of the cathode of the picture beam electron gun 9. These electrodes are so designed that the electric field between them brings a parallel electron beam to a focus in the symmetry plane, and the emerging beam is again collimated. The system is best regarded, therefore, as an electron lens system with a curved optical axis.

Figure 5 shows the equipotential lines, numbered in terms of V/ Vm, where Vm s the potential applied to electrodes 13, 13 and 14, and the potential of electrode 15 is assumed to be zero, i.e. the potential of the gun cathode. The figure shows also three electron trajectories. It is evident by inspection that if the incoming, collimated beam is focused in the symmetary plane, the outgoing beam will be again collimated and parallel to the original direction. A more accurate analysis reveals that in order to obtain the result it is not necessary to focus a wide beam exactly in one point of the symmetary plane. It is found that it is sufiicient if the focus of that thin bundle of trajectories which meets the symmetary plane at right angles is focussed in the said plane. The central trajectory of this thin bundle is the optical axis. It is found that trajectories which enter the lens at some larger distance from .the optical axis will meet the symmetary plane a little above its intersection with the optical axis, and at an incidence angle, relative to the normal, which may .5 be positive or negative. In first approximation the small height difference will be a quadratic function of the incidence angle. This, however, means that in this region a ray which meets the symmetary plane at a height h, at incidence angle |a, will continue at the other side as the ray which has the incidence angle -oc and will also emerge parallel to the symmetry plane. In other words, the present system, by its symmetry properties, has no second order error and therefore retains its property illustrated in Figure for quite wide beams.

Another valuable property is that a beam which is not collimated will emerge from the system with the same angle of convergence or divergence as that with which it entered. This follows from the fact that the system as shown in Figure 5 is what is known in optics as a telescopic system. Finally, thanks to the near optimum properties of the electric field shown in detail in Figure 5, a slight departure from the correct shape is of little consequence, and can be corrected by a small D.C. bias, applied to the electrode 15.

Strictly speaking, a cylindrical system will retain the properties described only for electron trajectories which are nearly parallel to the plane of the drawing in Figure 5. Electrons of an energy corresponding to Vm, which have been deected by the line detlector system 16, 16 (see Figures 12-15) at some angle a relative to the plane of the paper, will behave as if they had only the energy Vm cos2 a in that plane. They will be overfocused and will emerge as a somewhat diverging bundle. This eiect can be completely avoided if a voltage VIn sin2 a is applied to the electrode 15, so` that the voltage drop in the lens is Vm (l-sin2 o)=Vm cos2 a. This correcting voltage may be obtained, by means well known in themselves, by squaring the voltage applied between the line deectors 16, 16.

An alternative method of correction is to depart slightly from exact cylindricality of the electrode system, preferably by bending the central electrode 14 somewhat, so that its-distance from 15 is a little larger at the ends than in the middle of the mirror system.

The performance ofthe lens according to the invention can be improved for some purposes and particularly for use ina television cathode ray tube such as that described above with reference to Figures 1 to 4, by modifying the design in accordance with the principles now to be described with reference to Figures 6 to 11y of the accompanying drawings. The lens described with reference to Figures 1 to S acts as a plane mirror in the line MM of Figure 3 so far as concerns the path of the beam in planes parallel to the plane of the cathode ray tube screen. The deflection sensitivity of the system comprising the lens in combination with deflection plates 16 can, however, bc improved and certain aberrations of the system removed if the lens is made curved so as to behave as a convex mirror considered in planes parallel to the cathode ray tube screen.

In Fig. 6 .the three electrodes 13, 14 and 15 correspond to the elements similarly numbered in the previous figures. 15 is a trough-shaped repeller electrode, connected to the cathode of the tube in which the device is used, while the inner electrode or spine 14 and the outer or flanking electrodes 13 are connected to lthe most positive electrode in the tube. l

As explained above a cylindrical lens of this kind has a curved optic axis, which in the cross section of Fig. 6 appears as that ray, shown dot-dashed, which enters and leaves the lens parallel to its symmetry line at equal dis tances therefrom.A This may be called the telescopic property of the lens. lIt has -also been shown above that this curved line is, by reason of the symmetrical design of the lens, an optic axis in the same sense as the straight axis of an ordinary cylindrical lens. Rays entering the reversing lens parallel to the symmetry line, a little oi the optic axis, will not intersect the vertical symmetry line in exactly the saune/point as does the optic axis, but

a little higher, but in first approximation this point will be the same for two rays which are at the same small distance, right and left, from the optic axis. Consequently the second order error is zero for this lens for thin beams centered on lthe optic axis, and if they were originally parallel beams, they will leave the reversing lens again as parallel beams. More generally, all errors of even order vanish by reason of the symmetrical de sign. There remain, the errors of odd order, the first of which, the third-order error is usually referred to as the cylindrical abberation but this too can be reduced or eliminated. It may be noted that the telescopic property of the lens applies not only to beams which enter the lens parallel to the symmetry plane, but also to those at small angles thereto. In the following discussion the relative dimensions of lenses according to this part of the invention will be referred to in the following terms:

The distance A between the two branches of the optic axis `is taken as unity, and all other dimensions are referred to this.

W=The total outer width.

W1=The width of the repeller of negative trough 15.

W2=The width between the outer positive electrodes 13.

H=The height of the repeller.

G1=The gap between the repeller 15 bottom and edge of the central electrode or spine 14.

G2=The gap between the repeller and the outer electrodes.

W=The thickness of the centre electrode or spine r1=Curvature radius of the repeller, at the bottom.

r2=Curvature radius of the repeller, at the rim.

r3=Curvature radius of the flanking electrodes, at the rim.

R1=Radius of curvature of repeller bottom.

R2=Radius of curvature of spine edge.

R3=Radius of curvature of repeller edge.

R4=Radius of curvature of positive flanking electrodes.

Z=Distance of deflection centre from repeller bottom.

z=Distance of reversing line from repeller bottom.

All these dimensions are marked on the drawings.

Beams filling up to one-half of the free entrance and exit cross sections can be handled` Beyond this errors of 5th and higher order make their appearance.

By the telescopic property of the reversing lens, it will handle not only parallel but diverging or converging beams, without altering the convergence angle. If the entering beam converges in a point x units below the level Il" it will emerge so as to converge x units above it, with unchanged convergence angle. The line I- is the locus of the intersections of incident and emergent rays and will be referred to below.

In the lens according to this aspect of the invention, the electrodes are curved parallel to the plane in which the electron beamy is to be deected and the effect on the operation of the lens introduced by a convex curvature is illustrated in Fig. 7. In this figure and -in Fig. 8 the detlector plates by which the beam is deflected have been lgiven the same reference number, 16, as before.

As Figs. 7 and 8 show, the reversing lens acts in the XY-plane roughly speaking like a convex mirror. Though the initial` and the final rays, of which three are shown, intersect on a line I-l" which theory and experiments have shown to be almost straight, at a distance z below the bottom of the repeller, the angles are nevertheless much magnified, because the two rays will form very nearly equal angles with a line O'-O" at right angles to the bottom of the repeller trough. Consequently the angles a2 re larger than the corresponding angles a1, in the case shown about 3.3 times. Thus, with the same distance Z of the deflection centre from the repeller, a sweep angle of e.g. 113 is sufficient to cover a screen which with a straight reversing lens might require m30 or more.

This reduction of the primary deflection angles al has some important practical consequences. The first is an increase in deflection sensitivity, that is to say a reduction in the scanning voltage required for the horizontal sweep. This reduction is larger than expressed by the ratio of e.g. l3/30=0.43, because the sweep voltage increases more than linearly, almost quadratically, with the maximum deflection angle, owing to the fact that at larger angles the deflecting plates 16 must be spaced wider a art.

pAn even more important consequence is the reduction of the aberration known as deflection focusing or deflection astigmatism, that is to say the focusing of rays in the X-direction. It is known that, other things bein-g equal, this error increases with the square of the deflection angle, and is inversely proportional to the length of the deflector plates. By employing the device of the present invention there is no need to use very long plates 16. With deflecting plates of the shape as used in conventional cathode ray tubes the peak-to-peak scanning voltage for 13 deflection is only about one tenth of the beam voltage. Other things being equal therefore, the deflection aberration is decreased in the ratio of e.g. l3/3O2=0.19. A third consequence is that even the small remaining aberration may be partly corrected by the curved reversing lens, and can be entirely corrected or even over-corrected if desired, so that the improved lens can handle beams which are wider in the Xdirec tion too, not only in the direction at right angles.

The correcting effect can be understood from the fact that in the XY-plane the curved reversing lens acts approximately like a convex cylindrical mirror. It is known that the cylindrical aberration of convex mirrors is negative, that is to say parallel beams falling on it at increasing angles to the normal leave it with increasing divergence. This is true also of a lens according to this invention as has been confirmed by accurate graphical ray tracing, and also by direct experiments on reversing lenses as in Figs. 7 and 8. Consequently the overconvergence imposed on the beam by the X-deffection, due to deflection focusing is partly compensated by the divergence introduced by the reversing lens. Full compensation is also possible, if the distance Z (the distance of the deflection centre from the repeller) is made appreciably larger relative to the curvature radii R1 etc., of the lens than in the case shown in Fig. 7. One can, however, achieve full compensation or even overcompensation at any Z/R ratio, by curving the lens, or at least the negative trough 15, not into a circular arc but into a curve with a curvature increasing from the centre outwards, for example according to a fourth order law. Calculations show that full compensation can be achieved with a deviation from the circular shape which would only just be visible on the scale of Fig. 7.

The actual deflection astigmatism introduced by the X-deection plates 16 can, of course, be influenced by the choice of the deflection system. With the parallelfield deflection system shown in Figs. 7 and 8 the deflection astigmatism or over-focussing is only in the X direction, but is relatively severe, that is to say, the locus of the X focus is strongly curved. If, however, the plates 16 were replaced by deep channel-shaped deflection electrodes, the astigmatism in the X direction would be reduced. However, a similar aberration, but of opposite sign, would be introduced in the Y direction. That is to say, the beam would not focus in either the X direction or the Y direction at all points in a straight line, but the locus of the focus obtained in the X direction would be curved in one direction and the locus of the focus in the Y direction would be oppositely curved, so that focussing would not be achieved simultaneously in both the X and Y directions except in the centre of the sweep, that is in the absence of any deflection field. In Fig. 9 a system of deflection electrodes which achieves a useful compromise between these two situations is shown in perspective. It comprises two deflection plates 16 16" which are of shallow channel cross-section, and lying across the two open sides of the electrode system thus formed, and suitably spaced therefrom are two further plate electrodes 17' 17". These plates 17' 17 are maintained at the mean potential of the X deflection waveform. The effect then is that the beam is, as shown diagrammatically in the drawing, focussed simultaneously in both the X direction and in the Y direction at all angles of deflection, but the locus of the focus is less curved than in the case of the simple plate system of Figs. 7 and 8. Indeed, the locus is only half as strongly curved as in the parallelfield system. The use of such a deflection system enables the reversing lens according to the invention to be designed readily so as to eliminate the residual deflection astigmatism, without 4th order correction of the repeller profile.

In the design of the curved reversing lens according to the invention great care must be taken to ensure that a plane fan of rays entering it is reversed into a plane and not into a conical surface. It has been found that the shape of the said ray surface is appreciably influenced by the radius R2 of the spine, though at given R2 it is little influenced by the gap G1, that is to say by the spacing of the spine from the repeller bottom. It is found that with decreasing R2 the cone of emergent rays becomes concave towards the central plane, whereas with increasing R2 it becomes convex. R2 at which the spine is curved concentric to or slightly more sharply than concentric to the repeller bottom.

The curvature of said ray surface is also strongly influenced by the curvature radii R3 and R4 which determine the outer gap between the electrodes. It was found that it is convenient to keep G2 also uniform so that the remaining variable is the curvature of the outer gap, as a whole, if R1 and Z are considered as given. It was found that for Z/R1=0.7:-l.0 the best plane ray surface was obtained with a completely coaxial arrangement of the electrodes that is to say with the cross section as shown in Fig. 6 and in Fig. 8, rotated around an axis. If the curvature of the outer gap is made less than corresponding to coaxiality, the ray surface is a cone concave towards the symmetry plane of the lens; if the curvature is stronger, it becomes convex, that is to say it turns outwards. To a smaller extent the said curvature can be regulated by the gap G2, and this has the advantage that thereby a fine adjustment can be made without altering the shape of any of the electrodes. If the cone is concave towards the symmetry plane it can be a little flattened by reducing the gap G2 and vice versa. This, however, should be used only for small adjustments, as the beam quality deteriorates if G2 departs appreciably from its previously stated optimum value.

A further correction which may be applied to the lens s for compensating an effect which has been observed in the model as described, namely that the spacing A between the incident and emergent axes decreases somewhat towards the ends, and the lens, though telescopic for d=0, is not telescopic but convergent for larger deflection angles. This effect can be corrected by thickening the spine electrode 14 at its ends as shown in perspective in Fig. 10. In the example described the axis spacing varied from A=1.0 at the centre, to At=0.96 for a deflection angle a.1=0.1 radian and to A'=0.92 for a1=0.14 radian. This may be corrected by making the thickness of the centre spine electrode increase from w=0.3 A at the centre to w=0.6 A at each end, the increase in thickness being effected according to a parabolic law. The correct thickening for the spine may be achieved by shaping the two sides of the spine as conical surfaces of the appropriate radius and cone angle.

A second method for compensating the said reduction of the spacing A with increasing deflection angle consists in increasing the width W1 of the repeller trough with the deflection angle a, according to a parabolic law, for

It is plane at a certain.

instance from 2.0 in the centre to 2.05 where it is intersected by the ray deflected at a=0.1 radian, to 2.1 at a 0.14, and 2.2 at a=0.2. The repeller trough thus modiled acquires, seen from above, a shape roughly resembling that of a violin, as shown in perspective in Fig. 11. Both methods, that is to say thickening the spine towards the ends and widening the repeller trough, may be of course combined with advantage in order to keep A rigorously constant and independent of deection angle, and to ensure that the lens is telescopic at all deflection angles.

' As examples of lenses according to the invention the following table gives the relative dimensions of two designs which have given satisfactory results in terms of deflection sensitivity and freedom from aberrations, the dimensions being given in terms of the spacing between the incident and emergent axes as above stated:

Lens A Lens B W (Outer width of repeller trough) 2. 2. 34 W1 (Inner width of repeller trough) 2. 1 1. 96-2. 2 W2 (Inner spacing between outer positive electrod. 1. 75 1.66 H (Height of repeller in the central cross section) 0. 725 0. 72 G1 (Gap between repeller and spine) 0.51 0. 4 G2 (Gap betweenrepeller and pos. side electrode) 0. 525 0. 4 w (Thlckness of spine) 0.31 0.3-0.6 r1 (Radius of repeller prole) 0.3 0. 3 n (Radius of repeller edge). 0. 1 0. 1 r; (Radius of pos. anking electrodes profile) 0. 2 0. 1 R1 (Radius ol' repeller bottom) 8.0 5. 67 R2 (Radius oi spine edge) 8. 65 6.1 R3 (Radius of repeller edge) 8. 725 6. 4 R4 (Radius of postive tanking electrode 9. 25 6. 9 Z (Deflection centre to repeller trough) 7 6. 6 z (Distance of reversing line from rep. bottom measured) 1. 1 O. 75

Though in these designs all data have been related to the interplane distance A, it is understood that it is not necessary to scale up all dimensions when passing from one system to a larger one. It is possible, without large errors, to keep the cross section constant, and to scale up only the curvature radii R1 and the distance Z. Thel reason for this is that the electron motion as projected on the plane XY, as in Fig. 7, is in first approximation independent of the motion at right angles thereto, as shown in Fig. 6. Such an incomplete scaling-up will of course necessitate small adjustments in order to give a perfectly plane ray-surface.

No excessive accuracy is required in the manufacture of the electrodes of the reversing lens, which can be made by pressing, swaging or spinning from sheet metal, or by electro-forming. High-permeability soft nickel-alloys are preferable, as these act at the same time as magnetic screens.

I claim:

1. Electron lens for receiving an incident electron beam deflectable in an incidence plane and reversing it upon itself so as to return it in an emergence plane parallel to said incident plane and spaced therefrom comprising a trough-shaped repeller electrode presenting its open side towards the incident beam and spanning the space between said incidence and emergence planes with its side walls lying outside the respective planes, a spine electrode in the form of a wall lying between said incidence and emergence planes and having an edge lying in the mouth of said trough-shaped repeller electrode, and two anking electrodes each in the form of a wall and located one on each side of said spine electrode outside the respective incidence or emergence plane, the base of said repeller electrode and the edges of its side walls all being curved in a sense convex towards the direction of incidence, and the edges of said spine electrode and said lianking electrodes all being curved so as to define parallel gaps between the base of the repeller electrode and edge of the spine electrode and between the edges of the side walls of the repeller electrode and o f the llanking electrodes respectively.

2. Electron lens as claimed in claim l wherein the curvature of the base and edges of the side walls of said repeller electrode and the edges of said spine and flanking electrodes to the enumerated parts increases progressively from the centre towards each end of the lens.

3. Electron lens as claimed in claim 2`wherein the in crease in curvature follows a fourth-order law.

4. Electron lens as claimed in claim 2 wherein the spine electrode increases in thickness from its centre towards each end thereof.

5. Electron lens as claimed in claim 4 wherein the in crease in thickness of the spine electrode follows a parabolic law.

6. Electron lens, adapted to operate in conjunction with an electron gun and a beam deection system, having two symmetry planes at right angles to one another, the iirst of said planes being parallel to the plane fan of electron beams produced by said deflection system, while the second plane passes through the gun axis and through the mid-plane of the deliector system, comprising a trough-shaped repeller electrode of U-like cross section in planes parallel to the second-named symmetry plane, operative at a low potential near that of the gun cathode, a substantially plane, proled central spine electrode centered in the first-named symmetry plane, and two substantially plane, proiiled side electrodes, lianking said central electrode, the three last-named electrodes being operative at a high potential near the total accelerating voltage of the electron beam, all four electrodes having profiles in planes parallel to the first-named symmetry plane convex to the side from which the electron beam is incident, whereby the plane fan of the incident beams is converted into another plane fan of emerging beams, parallel to the first plane fan and symmetrical thereto relative to the first-named symmetry plane of the lens.

7. Electron lens as claimed in claim 6, in which the profiles of the spine and flanking electrodes and the profile in which the first-named symmetry plane intersects the repeller electrode are all substantially co-axial and consequently approximately equidistant along their length.

8. Electron lens as claimed in claim 6, in which the width between the lianks of the U-shaped repeller trough increases according to an approximately parabolic law with the distance from the second-named of the symmetry planes.

9. Electron lens as claimed in claim 6, in which the thickness of the spine electrode, at least on its portion facing the bottom of the repeller electrode, increases according to an approximately parabolic law with the distance from the second-named of the symmetry planes.

l0. Electron lens adapted to receive an incident electron beam detlectable in an incidence plane and return it in an emergence plane parallel to said incidence plane comprising a central spine electrode, a pair of flanking electrodes parallel to and disposed on opposite sides of said spine electrode in spaced relation thereto, the spaces between said flanking electrodes and said spine electrode defining incidence and emergence apertures, respectively, and a repeller electrode spaced from said spine and iianking electrodes and spanning said incidence and emergence apertures.

11. Electron lens as claimed in claim 10 wherein all of said spine and flanking electrodes are maintained at substantially the same positive potential and said repeller electrode is maintained atA a negative potential.

l2. Electron lens as claimed in claim 10 wherein said spine electrode defines a plane of symmetry of said lens, and said repeller electrode and the edges of said spine and lianking electrodes presented towards said repeller electrode are all curved in planes parallel to said plane of symmetry.

13. Deflection system for an electron discharge device comprising primary deflection means adapted to produce deflection of a cathode ray beam in an incidence 11 plane, and an electron lens adapted to receive an incident electron beam in said incidence plane and return it in an emergence plane parallel to said incidence plane, said primary deection means comprising a pair of deecting electrodes of channel cross section with the open sides of the channels facing one another across a gap and a pair of plate electrodes lying across but spaced from said gap, said electron lens comprising a repeller electrode, a central spine electrode presenting an edge 12 14. Deflection system as claimed in claim 13, wherein said plate electrodes are maintained at the mean potential of said deecting electrodes, and the gap between said defiecting electrodes is so dimensioned as to produce substantially equal deflection ffocusing in said incidence plane and in planes at right angles thereto.

References Cited in the tile of this patent UNITED STATES PATENTS towards said repeller electrode, and two anking elec- 10 2,178,238 Massa Oct. 31, 1939 trodes substantially parallel to and disposed on opposite 2,332,876 Uhlmann Oct. 26, v1943 sides of said spine electrode in spaced relation thereto 2,334,516 Szegho Nov. 16, 1943 and presenting respective edges towards the edges of Said 2,444,710 Ramberg July 6, 1948 repeller electrode, the gaps between the repeller elec- 2,536,878 Fleming Jan. 2, 1951 trode and the edge of said` spine electrode presented 15 2,554,170 Bruck May 22, 1951 thereto and between the edges of said repeller electrode 2,563,197 Sziklai Aug. 7, 1951 and the edges of said anking electrodes presented there- 2,759,117 Hasbrouck Aug. l14, 1956 to all being curved in planes parallel to said incidence 2,760,096 Longini Aug. 21, 1956 and emergence planes. 2,821,656 Foster Ian. 28, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2 926 274 February 23, 1960 Dennis- Gabor It is hereby certified that error appears in the printed specification of' the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 45, for "objection" read object column 4, line 39, after "13.", beginning with "The electron lens" and ending with "optical axis,"` in line 56, same column, should appear as a new paragraph instead of as in the patent; line 49, for "electode" read electrode column 6, line ll, for "abberation" read aberration v--; line 69, for "re" read are column 9,` in the table, column l thereof, fourth line from the bottom, for "postive" read positive column lO, line 4, after "electrodes'l strike out "to the enumerated parts".

Signed and sealed this 9th day of August l960 (SEAL) Attest:

KARL H. AXLINE Attesting vOfficer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT oEEICE CERTiFICATE OF CORRECTION Patent No. 2,926,274 l February 23, 1960 Dennis Gabor It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 45, for "objection" read object EL column 4, line 39, after "13.", beginning with "The electnon lens and ending with "optical axis," in line 56, same column, should appear as a new paragraph instead of as in the patent; line 49, for "electode" read electrode column 6, line ll, for "'abberation read aberration line 69 for "re" read are column 9, in the table, column 1 thereof, fourth line from the bottom, for "'postive" read positive column lO, line 4, after "electrodes" strike out t,o. the

enumerated parts".

Signed and sealed this 9th day of August 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Paten UNTTED STATES PATENT OEEICE CEETTEICATE OE CORRECTION Patent No.. 2,926,274 l February 23, 1960 Dennis` Gabor It is herebj)1 certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 45,l for .'objeetiow read Object -I;. column 4, line 39, after "13", beginning with "The electron lens and ending with noptical axisr in line 56, same column, should appear as a new paragraph instead of as in the patent; line 49, for "electodef read electrode --3 column 6, line ll, for "abberation" read aberration line 69j, for "rel read are column 9, in the table, Column l thereof, fourth line fromv the bottom, for "postivei read positive column lO, line 4., after "electrodes" strike out nto the enumerated parts".,

Signed and sealed this 9th day of August 1960 (SEAL) Attest:

KARL H'. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

